Transfer Hydrogenation of Alkenes Using Ethanol Catalyzed by a NCP Pincer Iridium Complex: Scope and Mechanism

The first general
catalytic approach to effecting transfer hydrogenation
(TH) of unactivated alkenes using ethanol as the hydrogen source is
described. A new NCP-type pincer iridium complex (BQ-NCOP)­IrHCl containing a rigid benzoquinoline backbone has been
developed for efficient, mild TH of unactivated C–C multiple
bonds with ethanol, forming ethyl acetate as the sole byproduct. A
wide variety of alkenes, including multisubstituted alkyl alkenes,
aryl alkenes, and heteroatom-substituted alkenes, as well as O- or N-containing heteroarenes and internal
alkynes, are suitable substrates. Importantly, the (BQ-NCOP)­Ir/EtOH system exhibits high chemoselectivity for alkene
hydrogenation in the presence of reactive functional groups, such
as ketones and carboxylic acids. Furthermore, the reaction with C2D5OD provides a convenient route to deuterium-labeled
compounds. Detailed kinetic and mechanistic studies have revealed
that monosubstituted alkenes (e.g., 1-octene, styrene) and multisubstituted
alkenes (e.g., cyclooctene (COE)) exhibit fundamental mechanistic
difference. The OH group of ethanol displays a normal kinetic isotope
effect (KIE) in the reaction of styrene, but a substantial inverse KIE in the case of COE. The catalysis of styrene
or 1-octene with relatively strong binding affinity to the Ir­(I) center
has (BQ-NCOP)­IrI(alkene) adduct as
an off-cycle catalyst resting state, and the rate law shows a positive
order in EtOH, inverse first-order in styrene, and first-order in
the catalyst. In contrast, the catalysis of COE has an off-cycle catalyst
resting state of (BQ-NCOP)­IrIII(H)­[O­(Et)···HO­(Et)···HOEt]
that features a six-membered iridacycle consisting of two hydrogen-bonds
between one EtO ligand and two EtOH molecules, one of which is coordinated
to the Ir­(III) center. The rate law shows a negative order in EtOH,
zeroth-order in COE, and first-order in the catalyst. The observed inverse KIE corresponds to an inverse equilibrium
isotope effect for the pre-equilibrium formation of (BQ-NCOP)­IrIII(H)­(OEt) from the catalyst resting
state via ethanol dissociation. Regardless of the substrate, ethanol
dehydrogenation is the slow segment of the catalytic cycle, while
alkene hydrogenation occurs readily following the rate-determining
step, that is, β-hydride elimination of (BQ-NCOP)­Ir­(H)­(OEt) to form (BQ-NCOP)­Ir­(H)2 and acetaldehyde. The latter is effectively converted to
innocent ethyl acetate under the catalytic conditions, thus avoiding
the catalyst poisoning via iridium-mediated decarbonylation of acetaldehyde.